Saturday, 4 December 2010

Icequake swarms portend some avalanches

Forecasting glacier crack-ups may be possible by keeping an ear to the ice
Unless you’re eating breakfast, hearing snap, crackle, and pop may be an early warning sign of an impending avalanche. Geologists listening in on “icequakes” that rumble through glaciers have developed a model that can predict a collapse up to 15 days before it happens, the team reports in a study posted on arXiv.org.
With that kind of heads up, villages could be evacuated and roads closed in avalanche-prone areas.
Though all glaciers groan and creak under stress, glaciers on an incline are especially creaky because gravity tugs on the top of the ice more than the base. Accumulating snow causes even more stress. These forces cause the glacier to fracture, sending tiny icequakes throughout. Eventually, if a glacier can’t handle the stress, a large chunk will fall off, pummeling any unsuspecting villages below with a moving mass of snow and ice.
To find early warning signs of a break-off, scientists in Switzerland placed seismic instruments on a glacier precariously hugging the northeast face of the Weisshorn, a mountain in the Swiss Alps that looms over the 400 inhabitants of the village of Randa, 2,500 meters below. Break-offs in the winter are especially dangerous because the glacier has accumulated snow, so that ruptures trigger avalanches. Weisshorn avalanches have claimed 51 lives since the 17th century.
The team traveled via helicopter in 2003 to plant the instruments — the glacier spans 3,800 to 4,500 meters above sea level on a slope of 45 to 50 degrees. The team also planted seven light reflectors mounted on stakes to help track the glacier’s movement, and left a camera across the valley to film changes in the dynamic landscape.
Researchers froze into the ice a special microphone, called a geophone, to pick up seismic vibrations. Two weeks before the glacier split in 2005, researchers were able to detect a change in the sounds picked up by the microphone.
“As you approach rupture, you hear more sounds,” says geologist and study coauthor Jérome Faillettaz of ETH Zurich. “It’s just like if you break a pen or a cracker. You hear some small noise before it breaks.”
Along with rumbling sounds, the team also saw the reflectors-on-sticks accelerate several days before the rupture. Scientists have known that seismic activity dramatically increases five days before a break-off, but by combining the motion of the glacier with the behavior of the icequakes, the researchers’ model can detect a rupture 15 days in advance.
“It’s the first time icequakes have been used as a precursor to these break-offs,” says glaciologist Fabian Walter of the Scripps Institution of Oceanography in La Jolla, Calif.
Though there are similar hanging glaciers all over the world, says Walter, few are near human settlements with lots of infrastructure.
Icequakes are less complicated to study than earthquakes because waves travel through only one medium, as opposed to several layers of the Earth. But just as scientists haven’t figured out how to predict earthquakes, predicting icequakes isn’t possible either.